DESCRIPTION: Mammalian cells respond to ionizing radiation-induced DNA damage by initiating transient delays in both G1 and G2 phases of the cell cycle. This protective response following DNA damage is hypothesized to allow cells to repair DNA damage prior to cells division. A number of laboratories have investigated the response to DNA damage induced by UV-C irradiation on transformed cell lines such as HeLa cells, rather than epidermal cells. However, UV-C radiation does not penetrate the earth's atmosphere, and UV-B is the radiation type relevant for human exposure. Therefore, experiments included in this proposal investigate the response of skin cells to UV-B irradiation. Previous studies in Dr. Pelling's laboratory have demonstrated that UV-B irradiation of mouse keratinocytes produced a 7-fold increase in p53 protein half-life and resulted in increased p53-DNA binding activity, accompanied by elevation in steady state levels of WAF/CIP1 mRNA, a cyclin/cyclin-dependent kinase inhibitor. Preliminary experiments exposing p53 knockout fibroblasts to UV-B showed that cells lacking both wild-type alleles of the p53 gene are incapable of inducing WAF/CIP1 expression following irradiation. In view of these observations it is hypothesized that keratinocytes are able to protect themselves from UV-B irradiation by undergoing a p53-mediated activation of WAF/CIP1 expression, which then produces cell cycle arrest by inhibition of cyclin-cdk kinase complexes. It is further hypothesized that epidermal cells from mice lacking wild-type p53 alleles will be more susceptible to UV-B irradiation-induced skin tumorigenesis. Experiments to test this hypothesis include studies to investigate whether a) UV-B irradiation induces WAF1 expression in keratinocytes and the possible role of p53 in mediating such induction (Aim 1); b) UV induction of WAF1 in keratinocytes is accompanied by a change in cell cycle (Aim 2); c) UV-B perturbation of the cell cycle is mediated by WAF1 inhibition of cyclin cyclin-dependent kinase complexes (Aim 3), and d) p53 knockout mice are more susceptible to UV-B skin carcinogenesis in vivo.